1. Field of the Invention
This invention relates to a metering valve for a fuel injection system and more specifically to an improved metering arrangement of such a valve.
2. Description of the Prior Art
Several different basic types of fuel injection systems have been devised. The most successful of the basic types have been the common rail system and the jerk pump system. Many variations and combinations of the basic systems have also been devised. The basic common rail system employs a single pump for providing high pressure fuel to a manifold and plurality of metering valves connected to the manifold. The rate of fuel metering in such systems may be constant or vary as a function of time even though the pump may provide a relatively constant pressure to the manifold and the metering valves. The basic jerk pump system employs one or more jerk pumps which provide both the injection pressure and the metering. The rate of fuel injection in such systems varies greatly with respect to time due to rpm variations of the engine crankshaft. Hence, the pressure varies greatly with respect to engine speed.
During the past several years, common rail systems have had decreasing success with compression ignition engines operating over a wide speed and load range. Compression ignition engines require high injection pressure. Several known types of metering valves capable of accurately metering high pressure fuel have required actuating forces which are relatively high and synchronized. Engine driven actuating mechanisms provided both. Such mechanisms also operate the valving members at speeds proportional to engine speed, i.e., increasing engine speeds causes increasing valving member speeds with respect to time, and have used rather complicated and costly metering arrangements to maintain and/or increase the quantity of fuel metered due to changing engine speeds and loads. Further, several of these prior art metering arrangements have employed means to vary the size of the metering passage or orifice through the metering valve to control the quantity of fuel metered, thereby varying the pressure drop across the valve member and reducing the pressure of the metered fuel at light engine loads.
Injection systems employing jerk pumps, which often combine pumping and metering into a single unit, have had a high degree of success with diesel engines. Such systems may have one combined unit supplying several engine cylinders via a distributor or one unit per engine cylinder. In either case, the unit often includes a piston and a bore defining a chamber which is expanded and contracted in response to reciprocating movement of the piston. The piston is reciprocated by an engine driven device, such as a cam, at speeds proportional to engine speeds. A variable volume of fuel is trapped in the expanded chamber and them impulsively pushed to an engine cylinder in response to the piston moving in a direction contracting the chamber. Such units have several disadvantages; High forces are required to raise the trapped fuel volume to the high injection pressures required for a diesel engine. The drive train between the piston and the engine must be designed to withstand high torques. If variable injection timing is required, the drive train must include a sturdy phase change mechanism capable of withstanding the high torques. The high driving forces cause side loading of the piston, thereby accelerating wear of the piston and the bore. Injection pressures are lower than ideal at low engine speeds and higher than ideal at high engine speeds, since the piston speeds are proportional to engine speeds. Leakage of fuel from the trapped volume increases with decreasing engine speed. And rise and fall of the injection pressure is rather slow due to the cyclic pumping of the fuel by the piston.
The mentioned copending patent applications disclose improved metering valves for high pressure, common rail fuel injection systems. These metering valves all employ valving members which require relatively low actuating forces, which momentarily define a continuous passage through the valve, and which define the continuous passage for a period of time independent of engine speed. In the metering valves of application Ser. No. 689,327, the engine throttle controls a sleeve for varying the cross-sectional area of the continuous passage through the valve; this method of controlling the amount of fuel metered adds cost and complexity to the valve and more importantly varies the pressure drop across the valve, thereby undesirably varying the pressure of the metered fuel at the valve outlet and to the engine injector nozzle. In the metering valve of applications Ser. Nos. 603,078 and 625,411, the throttle rotates or varies the angular position of the valving member in its bore to control a dump means which connects the valve output to a return at varying points while the continuous passage is defined, thereby dumping the outlet pressure to terminate fuel flow to the nozzle but also spilling fuel flowing through the continuous passage to the return. This dump method has proven to be very effective in controlling the amount of fuel metered to the engine and in sharply dropping fuel pressure to the nozzle to negate nozzle dribble; however, it has disadvantages in some applications since it also causes an increase in the amount of fuel provided by the high pressure pump and causes pressure fluctuations to the inlets of other metering valves supplied by the manifold.